1. Field of the Invention
The present invention relates to a secondary battery having a function to cut off a current path when the pressure in the battery rises, and a method of manufacturing such a secondary battery.
2. Description of the Related Art
In recent years, there has been a demand for secondary batteries, e.g., lithium ion secondary batteries, of high energy density which can be used for a long period of time through repeated charging and discharging cycles. It is more important than ever to take sufficient safety measures for those secondary batteries. Generally, secondary batteries are of a sealed structure. Therefore, for example, when a secondary battery is overly charged, the electrolytic large amount of gas in the battery, resulting in a sharp rise of the pressure in the battery. If such a condition continues, the battery itself tends to rupture, causing a dangerous situation. One of the safety measures which have been used in secondary batteries is a current cutting-off component hereinafter referred to as xe2x80x9cdisconnect devicexe2x80x9d) for cutting of a current path when the pressure in the battery rises.
One conventional disconnect device for use in secondary batteries will be described in detail below. As shown in FIGS. 1A and 1B of the accompanying drawings, a conventional secondary battery has a disconnect device disposed in an open end of outer casing 16. The disconnect device comprises rupture disk 11 having downward protrusion 11a with a flat tip end and closing the open end of outer casing 16, cap 10 electrically connected to an upper surface of rupture disk 11 by ring plate 12 and serving as an outer terminal of the battery, weld plate 14 serving as an inner terminal of the battery joined to the tip end of downward protrusion 11a and having vent holes 14a, and insulating gasket 13 sealing the gap between rupture disk 11 and outer casing 16 and holding rupture disk 11 and weld plate 14 in an electrically insulated fashion, except for a space around junction 15 between the weld plate and the rupture disk.
Insulating gasket 13 has a recess in which rupture disk 11 and cap 10 are mounted, an opening defined therein at the bottom of the recess, and a mount disposed in an inner peripheral surface of the opening and holding weld plate 14.
Weld plate 14 and rupture disk 11 are not electrically connected to each other except for junction 15.
Outer casing 16 holds therein a coiled assembly of an anode sheet, a cathode sheet, and a separator, and an electrolytic solution.
Weld plate 14 is of a circular shape. Electrically conductive tab 17 extending from the coiled assembly is welded to weld plate 14.
In the secondary battery constructed as the above, when the pressure in the battery rises, a stress is applied to rupture disk 11 in a direction to displace protrusion 11a toward cap 10, with a tensile force concentrated on junction 15 between weld plate 14 and rupture disk 11. When the tensile force exceeds the strength of junction 15, the tip end of protrusion 11a of rupture disk 11 disengages from weld plate 14, and protrusion 11a is deformed toward cap 10. The electric connection between the exterior and interior of the battery is broken, stopping the generation of a gas in the battery. As a result, the pressure in the battery is prevented from increasing.
The disconnect device should be designed such that when the pressure in the battery reaches a level tending to break the battery itself, the rupture disk will be peeled off, reliably cutting off the current path. The rupture disk is not permitted to remain undetached when the pressure in the battery reaches the level tending to break the battery itself. The rupture disk is not permitted to be detached easily due to shocks imposed when the battery drops onto a hard object.
Therefore, the mechanical strength of the junction between the weld plate and the rupture disk is an important factor for ensuring the desired reliability of the battery, and needs to be controlled at a certain value.
Japanese laid-open patent publication No.199106/1997 discloses a technique for reducing and stabilizing variations of welding strength. The publication describes that when a first plate (inner terminal plate 6) corresponding to a weld plate and a second plate (explosion-resistant valve body 3) corresponding to a rupture disk are welded, with an ultrasonic welding process, at a flat surface of the tip end of a protrusion of the first plate, variations of welding strength can be reduced and stabilized by limiting the area of the flat surface and the height of the protrusion which affect the welding strength.
However, the above publication only discloses that variations of welding strength can be reduced and stabilized by welding with an ultrasonic welding process joint surfaces whose area and shape are limited, but fails to reveal how a junction interface should be to control the peeling strength at a constant level.
When metal joint surfaces abut against each other and then the ultrasonic welding is performed, the area of intimate contact of convexities of all minute concavities and convexities on the metal joint surfaces increases, and the joining between the metal joint surfaces progresses due to diffusion of atoms at the surfaces that are held in intimate contact with each other. Thereafter, the diffusion of atoms activates the metal joint interface, eliminating voids to form molten metal regions (hereinafter referred to as xe2x80x9cnuggetsxe2x80x9d) in the metal joint surfaces. When nuggets are formed by the ultrasonic welding process, the peeling strength between the metal joint surfaces becomes equivalent to the material strength (shearing strength) of the regions where the nuggets are formed. Even with the technique disclosed in the above publication, when the diffusion of atoms progresses to the stage where nuggets are formed by the ultrasonic welding process, the peeling strength of the junction greatly depends on the material strength of the regions where the nuggets are formed, and is difficult to control merely by adjusting setting conditions for the ultrasonic welding process.
The peeling strength of the junction increases in proportion to the joined area. Therefore, when the joined area (current passing area) is increased in order to extract a large current from the secondary battery, if the metal joint surfaces are joined to achieve the same joined strength as the material strength, then the peeling strength of the junction is too large to cut off the current path.
As shown in FIGS. 1A and 1B, the electrically conductive tab extending from an electrode sheet is connected to the circular weld plate in the secondary battery. If forces are applied to the electrically conductive tab when the secondary battery is assembled or subjected to vibrations or shocks, the weld plate is liable to rotate in the joined surfaces. Since the weld plate is joined only in the junction, it produces torsional forces in the junction. Therefore, there is the problem in that the junction is unexpectedly peeled off.
It is an object of the present invention to provide a method of manufacturing a secondary battery so as to achieve a current passing area for extracting a large current from the secondary battery and also to easily control, at a constant level, the peeling strength of a junction between a rupture disk and a weld plate.
Another object of the present invention is to provide a secondary battery which has a highly reliable disconnect device including a junction that will not be detached when the secondary battery is assembled or subjected to vibrations or shocks, except when the pressure in the battery becomes abnormally high.
As a result of the inventor""s study conducted to achieve the above objects, it has been found that the peeling strength of a junction can be controlled by only setting conditions of an ultrasonic welding machine if the junction is produced by the diffusion of atoms in an initial states. And the inventors have invented a shape of joined surfaces in order to reduce and stabilize variations in the peeling strength after each joining. Furthermore, since the peeling strength of the junction is smaller than that of a completely diffusion-joined junction, the inventors have invented a construction of a disconnect assembly in order to prevent a rupture disk from being easily peeled off when the secondary battery is assembled or subjected to vibrations or shocks.
According to the present invention, there is provided a method of manufacturing a secondary battery having a disconnect assembly for cutting off a current path when a pressure in the battery is abnormal, disposed in an open end of an outer casing, the disconnect assembly including a rupture disk sealing the open end of the outer casing, a cap electrically connected to a portion of a surface of the rupture disk and serving as an outer terminal of the battery, a weld plate joined to a portion of another surface of the rupture disk and serving as an inner terminal of the battery and having vent holes, and an insulating gasket sealing a gap between the rupture disk and the outer casing and holding the rupture disk and the weld plate in an electrically insulated fashion, except for a space around a junction between the weld plate and the rupture disk, the method comprising the step of joining a portion of the rupture disk and the weld plate with an ultrasonic welding machine by finishing the diffusion of atoms at a junction interface in an initial stage.
With the above method, the peeling strength of the junction between the rupture disk and the weld plate is less than the strength of joined materials. Even if the area of junction is increased to extract a large current from the secondary battery, the junction achieved by the diffusion of atoms in the initial stage is free from a failure that the current path cannot be cut off due to an excessively large peeling strength. In addition, the junction achieved by the diffusion of atoms in the initial stage can easily be produced under setting conditions of the ultrasonic welding machine.
More preferably, in the above method one of the rupture disk and the weld plate having a protrusion and the other having a recess in which the protrusion is fitted, joined surfaces of the rupture disk and the weld plate are held in biting engagement with each other with the protrusion and the recess, and a portion of the rupture disk and the weld plate are joined with an ultrasonic welding machine by finishing the diffusion of atoms at a junction interface in an initial stage. The protrusion may be a hemispherical, columnar, radial, or striped protrusion. With this method, since the joined surfaces have the protrusion and recess that are held in biting engagement with each other, the joined surfaces can easily be positioned when they are joined. Because the joined surfaces are held in biting engagement with each other by concavities and convexities, the surface area of contact is increased so as to be equal or greater than the maximum projected area (area surrounded by the outer periphery of the junction) of the joined surfaces, so that variations in the occurrence of the initial diffusion-joined region per unit area of the joints are reduced. In this manner, variations in the peeling strength of the joints are reduced and stabilized. In addition, when a force tending to displace the joined surfaces therealong is produced, the concavities and convexities on the joined surfaces prevent the joined surfaces from being displaced relatively to each other. Consequently, the effect of the force applied along the joined surfaces on the joined stage is lessened.
In the above method, adjusting the progress of the diffusion of atoms in the initial stage is capable of adjusting the peeling strength of the junction between the rupture plate and the weld plate.
The initial stage comprises a stage where the area of diffusion-joined regions formed of nuggets is not in excess of 50% of joined surfaces.
The secondary battery manufactured by the above method has means for preventing the weld plate from being turned about the junction. Preferably, the secondary battery further comprises means for preventing the weld plate from swinging(rocking) about the junction. Since both the above means prevent the weld plate from being positionally displaced, stresses are not applied to the junction when the secondary battery is assembled or subjected to vibrations or shocks. Therefore, the reliability of the secondary battery is increased.
If the insulating gasket has a recess in which the rupture disk is mounted, an opening defined in a bottom of the recess, and a mount disposed in an inner peripheral surface of the opening for mounting the weld plate therein, means for preventing a turn of the weld plate and means for preventing a swing(rock) of the weld plate may be the following structure.
For instance, means for preventing the weld plate from being turned about the junction may comprise a tooth on an outer peripheral edge thereof, and the mount of the insulating gasket may have a groove in which the tooth is fitted. Alternatively, means for preventing the weld plate from being turned about the junction may comprise the weld plate which is of a polygonal shape, and the mount may be of a shape complementary to the polygonal shape of the weld plate. Means for preventing the weld plate from swinging about the junction may comprise the mount in the form of a groove gripping opposite surfaces of the weld plate.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.